1. Field of the Invention
This invention relates generally to a method and apparatus for assembly of liquid crystal cells and specifically to a method and apparatus for assembly of liquid crystal cells with a thin, uniform cell gap.
2. Background of the Related Art
The process for assembling liquid crystal cells includes a variety of steps. These steps are discussed in several references such as the book entitled "Liquid Crystal Flat Panel Displays--Manufacturing, Science & Technology" (ISBN 0-442-01428-7) which is incorporated herein by reference.
The major steps for assembling an active matrix display include: cleaning the substrate, applying and rubbing the liquid crystal orientation film (also called the alignment layer), applying the seal, placing the spacers, laminating and sealing the top and bottom surfaces, scribing the cells (if multiple cells are on a wafer or panel), injecting the liquid crystal, and sealing the injection hole.
The present invention is primarily concerned with the steps of applying the seal, placing the spacers, and laminating and sealing the top and bottom surfaces. The conventional method for performing these steps is described in the cited reference, "Liquid Crystal Flat Panel Displays--Manufacturing, Science & Technology."
Depending on the size of the substrate and final display size, a substrate may constitute several displays and an edge seal is needed around each display. The edge seal material can be applied using either silkscreening or dispensing. Typically, silkscreening is used.
If dispensing is used, the dispense time can be matched to other process times on an assembly line and multiple heads and sequential machines allow automation of the entire process. A further advantage of dispensing the adhesive, rather than screen printing it, is that contact with the inner surface of the display is avoided, and contamination and degradation of the aligning surface is greatly reduced.
Traditional edge seal adhesives, usually epoxies, have been heat cured after screen printing. The curing removes the solvent, and the resultant B-stage material is dry to the touch, allowing plate-to-plate alignment even when plates are in contact.
Just prior to lamination, spacers are deposited on one substrate to allow a precise gap between the top and bottom surfaces. Spacers may be fibers or spheres of a uniform dimension, made either from glass or plastic. The spacers are typically applied by air scattering. Typical large area flat panel displays have a cell gap of at least 5-10 .mu.m.
After alignment, heat and pressure are applied to cross-link the edge seal polymer. Pressure must be maintained during the cross-linking process so that proper spacing is achieved. Problems with this process include incomplete solvent removal, non-uniform pressure during curing, and movement of plates away from alignment as the seal deforms under pressure.
UV-cure epoxies have some advantages in an automated process, including the low viscosity needed for dispenser application. However, the adhesive must remain wet until the final UV curing step, which introduces some complications into the assembly process. The entire assembly equipment line must be kept in a controlled environment chamber, and plate-to-plate alignment must be accomplished without allowing the plates to touch.
After alignment, the plates are brought into contact, a sealing membrane is lowered, and the space between the plates is evacuated. Clamped together by the outside air pressure, the plates are moved to the curing station for UV exposure. After curing, the assembled plates are off-loaded into a cassette for liquid crystal injection.
FIG. 1 shows a conventional LCD 100 and FIG. 2 describes the conventional LCD assembly process. LCD 100 includes a top substrate 110, a bottom substrate 120, a top alignment layer 112, a bottom alignment layer 122, an edge seal 130, and spacers 140. It is well known to those skilled in the art that a variety of different materials may be used for each of these elements depending on the particular application. For example, top substrate 110 can be a common electrode such as Indium Tin Oxide (ITO) on glass. Bottom substrate 120 can include active circuitry for implementing the pixels of an active matrix LCD or patterned ITO in a passive matrix LCD. Alignment layers 112 and 122 can be a rubbed polymer or evaporated SiO.sub.x. Edge seal 130 can be an adhesive, such as an epoxy, which is applied by screen printing or dispensing. Spacers 140 can be glass microspheres which are blown or spun on to one of the substrates to ensure a uniform cell gap.
FIG. 2 shows a method for assembling conventional LCD 100 of FIG. 1. The method begins with step 210 which involves depositing alignment layers 112 and 122 on top substrate 110 and bottom substrate 120, respectively. Step 210 is followed by step 220 which involves applying edge seal 130 to either top substrate 110 or bottom substrate 120. Edge seal 130 can be applied either by screen printing or dispensing and defines the optically active area of conventional LCD 100. The method continues from step 220 to step 230 which involves applying spacers 140, by blowing or spinning, to either top substrate 112 or bottom substrate 122. The method progresses from step 230 to step 240 which involves attaching top substrate 110 and bottom substrate 120 with uniform pressure to achieve the desired cell gap. The method continues from step 240 to step 250 which involves curing edge seal 130. The method progresses from step 250 to step 260 which involves filling the cell with liquid crystal. The method continues from step 260 to step 270 which involves sealing the cell.
There are numerous disadvantages related to the traditional method of assembling LCDs as shown in FIG. 2. Traditional LCD assembly techniques, such as microsphere spacer application and screen printing glue are not conducive to the assembly of high-definition miniature LCDs. As the pixel pitch decreases below 20 .mu.m, microsphere clustering and diameter nonuniformities can result in severe performance degradation.
Furthermore, the screen printing or dispensing of an adhesive edge seal on the substrate is a technique borrowed from the flat panel community. The edge seal tends to bleed (spread out) and is hard to apply precisely resulting in glue encroachment onto the aperture. The amount of bleeding is typically two to three times the originally printed width. As the width of the edge seal is decreased, the effects of glue bleed become more pronounced. This translates into wasted silicon area and/or poor assembly yield. The trade-off between circuit and assembly yield is compromised by limiting the number of displays that can be fabricated on an individual wafer.
Additionally, the separate steps of applying the edge seal and applying the spacers increases the manufacturing complexity and reduces the uniformity of the process. This disadvantage is particularly important in the assembly of miniature LCDs which require a small cell gap. Obtaining uniform performance of large area displays also requires that the cell gap between the two plates be very closely controlled. This is especially true for STN displays, where the cell gap may be only 5 .mu.m and gap tolerance is .+-.100 nm or even less. This introduces extreme requirements for uniformity of spacer size and for reproducibility of positioning the plates prior to curing.
There have been numerous attempts to solve problems associated with LCD assembly. For example, Gurtler (U.S. Pat. No. 3,909,930 issued Oct. 7, 1975) discloses a method for fabricating a liquid crystal display device involving providing a photopolymeric layer (a polyester with a photosensitizer added) with a cavity in the middle for electrodes. Gurtler teaches the use of photopolymeric material, such as RISTON, to achieve a cell gap of 12.5 .mu.m (0.0005 inches). Gurtler specifically discloses the heating of the photopolymeric material u4ntil it flows to seal the edges.
Unfortunately, Gurtler is not able to solve the above-mentioned problems because the selection of materials such as RISTON does not provide the thin, uniform cell gap required in modern LCD assembly. A cell gap of 12.5 .mu.m along with the bleeding of the edge seal material is simply not acceptable in many modern applications, such as the assembly of miniature LCDs. These applications typically require a small cell gap, e.g. &lt;4 .mu.m, within a tight tolerance such as .+-.100 nm, and with no bleeding of the edge seal material.
Swirbel (U.S. Pat. No. 5,515,191 issued May 7, 1996) and Williams et al (U.S. Pat. No. 5,378,298 issued Jan. 3, 1995) are more recent attempts at solving some of the above-mentioned problems. Swirbel discloses an LCD with spacer material 27 formed on metal conductors 16 which are connected to a series of elements or pixels 13 as shown in FIGS. 1 and 3. Williams et al complements Swirbel by disclosing that the spacer material 27 can be made of an acrylic adhesive (such as LOCTITE 352) which is UV curable. Williams et al evaluates a variety of materials for use as the adhesive material and displays the results in tabular format. Example 1 uses LOCTITE 352 as the adhesive resulting in a cell gap of 10-12 .mu.m.
Both Swirbel and Williams et al, evaluated either alone or in combination, still fail to solve all of the above-mentioned problem. A cell gap of 10-12 .mu.m is simply too large for many modern applications such as assembly of miniature LCDs. These modern applications require a small cell gap within a tight tolerance, and with no bleeding of the edge seal material.
A method for easily and inexpensively assembling liquid crystal cells with a small cell gap, for example &lt;4 .mu.m, will result in the following advantages. Any reduction in the cell gap is directly related to the field strength across the liquid crystal. For example, in applications requiring a low operating voltage for the LCD, if the cell gap is decreased, the operating voltage for the LCD can be decreased while still providing the same field strength to the liquid crystal. This is a significant advantage for applications involving miniature LCDs, since these devices are frequently portable and battery powered.
On the other hand, some applications require a stronger field to allow faster switching time. For example, a portable color LCD TV with separate RGB frames presented sequentially requires a 180 Hz frame rate (3.times.60 Hz). A stronger field allows faster switching, but it also consumes more power. However, if the power is held constant, the field strength can be increased by simply decreasing the cell gap. This allows the use of portable, battery-powered LCDs in applications requiring fast switching times.
Additionally, the field within the liquid crystal is related to the resolution of the LCD because of the effects of fringing fields between pixels. This effect is particularly important in high resolution applications in which the pixels may be spaced at distances less than 1 .mu.m. If the cell gap is decreased, the LCD will have a reduction in field interference between neighboring pixels which allows the pixels to be more closely spaced. This closer spacing of pixels produces a higher resolution image on the LCD.
In addition to reducing the cell gap size to &lt;4 .mu.m, it is also important to reduce the deviation of the cell gap to .+-.100 nm. Any greater deviation causes a reduction in the resolution and contrast of the LCD. This occurs because the lack of uniformity in the cell gap across the display causes different field strengths at different pixels even if they are provided with the same input power. This is particularly apparent in applications in which a solid image (with equal voltages at all of the pixels) is displayed across the entire LCD. If the LCD has a non-uniform cell gap, the perceived image will have dark and light spots depending on the field strength. This effect is even more significant in reflective displays since the effects of the non-uniformities are doubled.
Furthermore, it is advantageous to reduce the amount of bleeding (or spreading) of the edge seal. This is important because using an edge seal which can be applied precisely without bleeding results in maximization of the available die space for imaging.
Because of these advantages, there is a need for a new method and apparatus for assembly of liquid crystal cells with a thin, uniform cell gap.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.